There are many different design programs on the market that can help you design and build model aircraft. Most cost $50 or more, but we'd like to show you something that is considerably less. Sometimes you get more than you pay for!

Before we discuss Profili 2.11, we need to mention another program: Xfoil. It was written by Mark Drela of the Massachusetts Institute of Technology and is an interactive program for the design and analysis of subsonic isolated airfoils. Xfoil is a freeware program available at http://raphael.mit.edu/xfoil. Although it's good, it was written in the days of DOS and can be tricky to run if you're not used to typing in commands from a C:> prompt.

Enter Profili 2.11, written by Stefano Duranti, which does a masterful job of dragging XfoiI from the computer dark ages into a modern mouse-driven, point-and-click system. The program requires Windows 95/98/Me/XP or NT 4.0/2000 and 50MB of free disk space. It uses the excellent basic Xfoil Computations but has added an interface that makes the program fun to use.

Profili 2.11 can be downloaded from Stefano's Internet site, and he is asking for a $10 fee to register the program. A CD-ROM version can be ordered for $15, which includes shipping. Stefano has only one fee, even if you put the program on several computers, but he would like you to register all of them. It is a realistic policy, and he is to be commended for his decision.

On the Profili home page there are long lists of amazing things the program can do for you, but we can't include them here because of space limitations; however, following is a little of what we found about it.

Profili has a database of more than 2,200 airfoils with precomputed aerodynamic characteristics, which really saves time in making airfoil comparisons. The user's ability to easily change the airfoils slightly (or even extremely) has been made effortless. The program will then compute the new aerodynamics resulting from your changes. It is interesting to watch the computations take place; you gain a "feel" for how the pressures are distributed over and under the airfoil.

Once you input your pet airfoil, Profili goes to work. Depending on the complexity of the airfoil (maybe something exotic such as a double diamond shape with a square leading edge and ripples across the bottom), it may look as if it's locked up. Normally this is because Xfoil is having trouble with the pointy edges of your pride and joy. This is not a failure in Xfoil or Profili; there are just limits to what is possible with computational fluid dynamics.

To help manage this, Profili has an adjustable time-out function to keep things moving. When you try to design the world's most marvelous airfoil, keep in mind that nature prefers smoothness and will reward a finesse approach to airfoil design.

You might have heard the local theorists at the flying field discuss Reynolds numbers (Re). Profili will compute Re for you when you input your projected flight speed and model size. The accompanying screen shots are examples of selecting a particular airfoil, in this case an HQ 3.0/15, and calculating the aerodynamic characteristics at different Res. One thing we noticed was that for the HQ airfoil, as well as many others, there is a lot of variation in drag coefficient (Cd) with Re. It is one of the reasons big models seem to fly better than small ones.

Did you ever wonder what something like tabulators did to help or hinder an airfoil? Profili will let you investigate how things such as these can help (or hinder) the aerodynamics of any wing, but these examples are only scratching the surface of what can be learned from using Profili.

Profili has a built-in "Genius" that senses when you need help and will suggest things for you to do to help smooth your designing sessions. Although you can turn off the Genius, it is handy and accurate.

After the final aerodynamic calculations are finished and you have refined what you need for your design, Profili is able to help even more. It can create and print rib templates to use as guides for foam cutting

or print standard ribs to allow conventional balsa construction. You can even export your final ribs as a DXF file to import into other CAD software.

You can select wing-taper ratios or even elliptical planforms, leading-edge sizes, spar shapes and sizes, lightening holes, trailing-edge widths, and skin or capstrip thicknesses.

Figure 8 is an example of the rib templates for some HQ airfoils. The taper of the wing, the airfoil type, and the twist of the wing were selected, and Profili produced templates that incorporated all of them. The program can put tabs on the lower surface of the ribs to allow the wing to be built on a flat surface, regardless of the airfoil contours.

Profili 2.11 is a good program for its educational potential and for the way it can ease the model-aircraft design and building process. The fact that it automatically generates rib templates, with just a little input from the user, can save tons of time. And don't forget about the bang for the buck; this program does a great job with a minimum amount of fuss, while leaving money in your wallet.